app_shell: Add ShellContentRendererClient

extensions/common/extensions_client.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/basictypes.h"
+#include "base/logging.h"
 #include "extensions/common/extensions_client.h"
 
 namespace extensions {
 
 namespace {
 
 ExtensionsClient* g_client = NULL;
 
 }  // namespace
 
 ExtensionsClient* ExtensionsClient::Get() {
+  DCHECK(g_client);
   return g_client;
 }
 
 void ExtensionsClient::Set(ExtensionsClient* client) {
   // This can happen in unit tests, where the utility thread runs in-process.
   if (g_client)
     return;
   g_client = client;
   g_client->Initialize();
 }
 
 }  // namespace extensions